Widespread use of a web search system, such as Google (registered trademark), has made commonplace a full text search system for electronic files. With a search system, the user searches for information on a daily basis.
Today, the major trend in the full text search systems for electronic files is that an index for high-speed search is created in advance and a user searches an electronic file using this index. Examples of such information search systems are described in Patent Document 1 and Non-Patent Document 1.
The N-gram based method and the morphological analysis method are known as a technology for creating an index (Non-Patent Document 1).
An index created by those methods, which is generally uncompressed, requires a large capacity that is about 200-300% of the capacity of the original documents.
Another problem is that it is difficult to update this index dynamically.
A user searches for information using this index. This means that the user can get the search result of search target data existing at the time the index was created or updated.
For example, if the index is created based on one-day-old search target data, additions or updates that are made after the index creation time for the search target data are not reflected on the search result.
So, there is a need for reflecting search target data on an index as real-time as possible. The technologies for satisfying this need are described in Patent Document 1 and Patent Document 2.
Patent Document 1 discloses a document search device that, with the index of newly registered document files created in a memory, the document search unit uses both the index data on the new document data stored in the memory and the index data on existing document files stored in the disk device to search the document files. Patent Document 2 discloses a document search device that registers index data on new documents in a sub-index smaller than the main index when the new documents are registered in the index and, at a search time, accesses both indexes to combine the results into the search result.    Patent Document 1: Japanese Patent Kokai Publication No. JP-A-9-223152 (page 1)    Patent Document 2: Japanese Patent Kokai Publication No. JP-A-7-146880    Non-Patent Document 1: Kita Kenji, Tsuda Kazuhiko, Shishibori Masamiki “Information Search Algorithm”, KYORITSU SHUPPAN CO., LTD, Jan. 1, 2002, pp. 6, pp. 160-179